1. Field of the Invention
The present invention relates to capacitors used in diverse electronic devices, and more particularly to electrode foils for wound capacitors using conductive polymer for their solid electrolyte, and solid electrolytic capacitors using this type of electrode foil.
2. Background Art
In line with the increasing use of high frequencies for electronic devices, electrolytic capacitors, one type of electronic components, are also expected, more than ever before, to have large capacitance and good impedance characteristics in a high frequency range. To meet these demands, various solid electrolytic capacitors using conductive polymer with high electric conductivity as solid electrolyte have been studied.
To satisfy the demand for large capacitance, wound solid electrolytic capacitors (an anode foil and a cathode foil are wound with a separator in between) using conductive polymer as solid electrolyte have been commercialized. A winding structure can achieve large capacitance more easily compared to a structure of stacked electrode foils.
This type of solid electrolytic capacitors demonstrate good service life, good temperature characteristic, and in particular, excellent high frequency characteristic, and are thus broadly employed in power circuits of personal computers. Now, a technology of using a nonvalve metal foil as the cathode foil, such as a nickel foil, that hardly forms natural oxidation film is proposed with the aim of increasing electrostatic capacitance C. With this technology, the electrostatic capacitance of the cathode becomes substantially infinite. This technology is disclosed in Examined Japanese Patent Publication No. H4-7086.
A plain foil of nonvalve metal cannot be roughened by etching. Accordingly, an effective contact area between the cathode foil and solid electrolyte is reduced, generating a large equivalent series resistance in a finished capacitor. Therefore, a technology of using the cathode foil configured in a different way has been proposed. A surface of an aluminum foil is roughened by etching, and then nickel, which is nonvalve metal, is plated on this roughened surface by electroless plating. This technology is disclosed in the Japanese Patent No. 3439064.
The above conventional solid electrolytic capacitor uses the nickel foil as the cathode foil. However, the nickel foil is more expensive compared to the aluminum foil, which is generally used for the cathode foil. Futhermore, the conventional electroless-plating method used for plating nickel on the surface of metal foil, whose surface is roughened by etching, is not capable of further thinning a plating film. Accordingly, it is difficult to uniformly plate the nickel film into fine pores on the roughened surface. In addition, 1-μm thick film will fill etching pits. Furthermore, an oxide film that exists on the surface of the roughened metal foil hinders firm bonding between the metal foil and the nickel plating film.